We have an aging but well maintained HP LaserJet 5200 that has developed a "light" printing problem. Pages are being printed significantly lighter than they used to. We had just replaced a toner so I tried another one and the same problem (and actually had a dark streak down the left side of pages. So, I put the old toner back in.

In all instances, the printer would display SLOW MODE. I looked that up and saw it referred to the fuser and how it may need to cool between prints. However, this happens even on just 1-2 page prints as well.

So, would this SLOW MODE cause lighter printing issues? Even though it has only been 8 months since the last fuser replacement, is it time already, considering the age and moderate volume we have on this printer?

Good job, it confirms that some circuits in the printer operates the way they should. Important clue. Best would be if you could post a picture for density 2, 3 and 4.

Get bottle or Vodka or Absinthe and clean the laser window. Remove toner/developer assembly. In the ceiling is a long narrow glass window across the entire printer. Make sure contacts on the side of the assembly and opposing parts are clean, no scattered toner. The contacts are various metal pieces and wires protruding from the assembly sides.

So far it could be developer, laser/scanner assembly, hight voltage power supply for corona.

Be certain that paper type, weight, size is set correctly in the printer and driver. The setting has direct influence on developer function.

Ding ding ding ding ding! Low (well, lower than it needs to be) voltage will definitely cause light printing. The corona wire needs to be sufficiently charged so it can properly charge the drum surface. There is also another corona wire that sits behind the paper that draws the toner from the drum surface to the paper.

Anyway, everything related to using the high voltage is in the toner cartridge. If you replaced the toner cartridge and still got a light print, then it is more likely that the power supply is failing to provide sufficiently high voltage to charge the drum properly.

t511 also mentioned cleaning the laser glass. However, unless someone sneezed and blew toner all around the inside of the printer, it is unlikely that toner would cover this glass in a uniform fashion such that you would not see noticeable vertical streaks of light print.

Unless you are a printer tech (and I expect that to not be the case); if a fresh toner cartridge does not clear this up, you should have this printer serviced professionally.

Don't forget the easy stuff. I've seen transfer rollers cause this issue. You are looking at anything from voltage/power supply. You can start by testing the voltage that gets to the printer. Cleaning the lenses and mirrors in the laser scanner is another good idea that was mentioned. But more often, as far as the printer goes, transfer rollers cause this quite a bit. The one other thing I didnt notice if anyone mentioned was the toner itself. I have seen the actual toner used in cheap recycled cartridges be problematic. I did printer and copier repair for 12 years. I have seen bad toner cause lots of issues, even not fusing properly. Start easy - if you have another HP LJ 5200 swap the cartridges and see if the problem stays with the printer or follows the toner cartridge.